Variable flowrate pumps can be automatically controlled by providing electrical signals indicating desired flowrate. When the pump is a positive volume displacement pump such as a peristaltic pump, a signal to move the pump actuator (e.q., an electrical motor) a specified amount should result in pumping the corresponding volume of liquid. Under some conditions a pump may not pump the commanded amount. E.g., when pumping at a low flowrate and thus low motor speed, the motor may cog, rotating in increments in an unsmooth manner, with resulting loss in accurate flow control. Some other examples are when the pump is commanded to pump at a speed above its upper limit, or when pump operation is impeded, e.g., by jamming or restricted movement of pump parts.